In a typical imaging device, light enters through an opening (aperture) at one end of the imaging device and is directed to an image sensor by one or more optical elements such as lenses. The image sensor consists of pixels that generate signals upon receiving light via the optical element. Commonly used image sensors include charge-coupled device image sensors (CCDs) and complementary metal-oxide semiconductor (CMOS) sensors.
Conventional digital cameras typically achieve color separation by performing color separation in the optical path and using a separate image sensor for the wavelengths of light corresponding to each of the primary colors (i.e. RGB), using an image sensor with color separation and multiple signal collection capability within each pixel, or by applying filters over a single sensor so that individual pixels detect wavelengths of light corresponding to one of the primary colors. Use of filters is particularly common in cameras that have a small form factor, such as cameras incorporated in mobile phone handsets and other consumer electronics devices including but not limited to, laptop computers and televisions. A common filter that is formed on image sensors is the Bayer filter, the pattern of which includes 50% green filters, 25% red filters, and 25% blue filters. The output of an image sensor to which a Bayer filter is applied can be reconstructed as a color image using interpolation techniques.
Image sensors are subject to various performance constraints including, among others, dynamic range, signal to noise (SNR) ratio and low light sensitivity. The dynamic range is defined as the ratio of the maximum possible signal that can be captured by a pixel to the total noise signal. The SNR of a captured image is, to a great extent, a measure of image quality. In general, as more light is captured by the pixel, the higher the SNR. The light sensitivity of an image sensor is typically determined by the intensity of light incident upon the sensor pixels. At low light levels, each pixel's light gathering capability is constrained by the low signal levels incident upon each pixel.
A challenge associated with increasing the number of pixels in an image sensor is that the lens system is dimensioned to span the image sensor. The problem is most acute with mobile cameras, such as those used in mobile phones and consumer electronics devices, where the form factor of the lens system can significantly impact the overall form factor of the mobile device.
In response to the constraints placed upon a traditional digital camera based upon the camera obscura, a new class of cameras that can be referred to as array cameras have been proposed. Array cameras are characterized in that they include multiple arrays of pixels, each having a separate lens system. Examples of 2, 3 and 4 array cameras in which each array of pixels captures light from a different band of the visible spectrum and the captured images are combined to create a full color image is disclosed in U.S. Pat. No. 7,199,348 to Olsen et al., the disclosure of which is incorporated by reference herein in its entirety. U.S. Pat. No. 7,262,799 to Suda, the disclosure of which is incorporated herein by reference in its entirety, discloses a 2×2 array camera including one sensor used to sense a red (R) image signal, one sensor used to sense a blue (B) image signal and, two sensors used to sense green (G) image signals.